In test chamber 16, there is a turret inside a ventilation grate right past the elevator. Jump over it, get the refraction cube and use the laser to destroy that turret. The grate will open up, and when you go in, there will be 4 turrets singing a song in unison.

Complete the following tasks to unlock the corresponding achievement. To view your achievements and stats in Steam, select "Community", then "My profile", then "View all my games", then the game and view stats.

Achievement

How to unlock

Air Show

Perform 2 aerial gestures before touching the ground in co-op.

Asking for Trouble

Taunt GLaDOS in front of a camera in each of the five co-op courses.

Bridge Building

Complete all test chambers in the Hard-Light Surfaces co-op course.

Bridge Over Troubling Water

Complete the first Hard Light Bridge test.

Can't Touch This

Dance in front of a turret blocked by a hard light bridge in co-op.

Confidence Building

Complete all test chambers in the Mass and Velocity co-op course.

Door Prize

Examine all the vitrified test chamber doors.

Drop Box

Place a cube on a button without touching the cube.

Dual Pit Experiment

Do the same test twice.

Empty Gesture

Drop your co-op partner in goo while they are gesturing by removing the bridge under them.

Final Transmission

Find the hidden signal in one of the Rat Man's dens.

Four Ring Circus

Enter 4 different portals without touching the ground in co-op.

Friends List with Benefits

While playing co-op, hug 3 different people on your friends list.

Gesticul-8

Perform all 8 gestures of your own volition in co-op.

Good listener

Take GLaDOS' escape advice.

High Five

Celebrate your cooperative calibration success.

Iron Grip

Never lose a cube in Chamber 6 of the Mass and Velocity co-op course.

Lunacy

That just happened.

Narbacular Drop

Place a portal under your co-op partner while they are gesturing.

No Hard Feelings

Save a turret from redemption.

Obstacle Building

Complete all test chambers in the Excursion Funnels co-op course.

Overclocker

Complete Test Chamber 10 in 70 seconds.

Party of Three

Find the hidden companion cube in co-op test chamber.

Pit Boss

Show that pit who's boss.

Portal Conservation Society

Complete Chamber 3 in the Hard-Light Surfaces co-op course using only 5 total portal placements.

Portrait of a Lady

Find a hidden portrait.

Preservation of Mass

Break the rules in Test Chamber 07.

Professor Portal

After completing co-op, complete Calibration Course online with a friend who hasn't played before.

Pturretdactyl

Use an Aerial Faith Plate to launch a turret.

Rock Portal Scissors

Win 3 co-op games of rock-paper-scissors in a row.

SaBOTour

Make a break for it.

Scanned Alone

Stand in a defective turret detector.

Schrodinger's Catch

Catch a blue-painted box before it touches the ground.

Ship Overboard

Discover the missing experiment.

Smash TV

Break 11 test chamber monitors.

Stalemate Associate

Press the button!.

Still Alive

Complete Course 4 with neither you nor your co-op partner dying.

Stranger Than Friction

Master the Propulsion Gel.

Tater Tote

Carry science forward.

Team Building

Complete all test chambers in the Team Building co-op course.

The Part Where He Kills You

This is that part.

Triple Crown

Solve 3 co-op chambers in the Mass and Velocity course in under 60 seconds each.